The present invention relates to a receiver and, in particular but not exclusively, to a RAKE receiver for use in a cellular telecommunications network. The cellular telecommunications network may, but not necessarily, use code division multiple access (CDMA).
When a signal is transmitted from a base station to a mobile station or vice versa, the signal will follow a number of different paths (multipaths) due reflections of the signals from buildings or the like. Thus the same signal will arrive at its destination mobile station or base station at different times, depending on the length of the path travelled. RAKE receivers are known and are generally used to resolve this problem. In RAKE receivers, a different one of the propagation paths is assigned to different fingers of the RAKE receiver and these signals are then combined to provide a single signal. However, if the mobile station is moving or if vehicles or people are moving near the mobile or base transceiver stations, this will result in changes in the relative phases of the different multipath signals. This in turn causes the power of the combined single signal to fluctuate. Without these movements the channel impulse response would remain generally constant. Accordingly, the rate of change of the channel impulse response is related to the speed of the movements mentioned hereinbefore.
Relative movement of the base and mobile stations causes Doppler shifts in the various multipath signals which gives rise to Doppler spread in the received signal. This can be viewed as spreading of the transmitted signal frequency. The Doppler spread in the received signal is related to the rate of fluctuations in the received signals. The reciprocal of the Doppler spread is the coherence time of the channel which is the time interval over which a transmitted symbol will be relatively undisturbed by channel fluctuations. The relative speed of a mobile station relative to a base station provides a measure of the coherence time of the channel.
In known RAKE receivers, each finger includes a smoothing filter. However the characteristics of these filters are fixed. This gives rise to the problem that the RAKE receiver only provides optimum results when the coherence time of the channel is within a limited range of values. This means that with some coherence time values, the output of the RAKE receiver is degraded as a result of the poorer filtering by the smoothing filter.
In a code division multiple access system, soft handoff is used. With soft handoff, a mobile station communicates with more than one base station at a time. Different fingers of the RAKE receiver may therefore be allocated to receive signals from different base stations. Accordingly the coherence time of the channels for the signals from the two different base stations may be quite different. According at least one of the signals from one of the base stations may not be processed in an optimal manner. This means that the quality of the combined signal may be reduced.
It is therefore an aim of embodiments of the present invention to reduce or at least mitigate the problems mentioned hereinbefore.
According to one aspect of the present invention, there is provided a receiver for use in a wireless communication system, said receiver comprising a plurality of receiver means, said plurality of receiver means each being arranged to receive signals from a different propagation path, each of said receiver means comprising means for estimating the channel coherence time for the propagation path used by signals received by the respective receiver means and filtering means, wherein the operation of the filtering means is altered in dependence on the coherence time estimate provided by the estimating means.
As the filtering means is altered in dependence on the coherence time estimate provided by the estimating means, the filtering operation provided by the filtering means can be optimized to reduce the effects of noise.
Preferably, the receiver is a rake receiver and said receiver means comprise fingers.
Preferably, tap coefficients for said filtering means are alterable in dependence on the coherence time estimate. Additionally or alternatively, the number of taps used by said filtering means is alterable in dependence on the coherence time estimate. In this way, the operation of the filtering means can be altered.
Preferably, the filtering means has the characteristic that the mean square error of the signal is minimised. The filtering means may therefore comprise a Wiener filter.
However, other types of filter can be used. For example, the filtering means of each receiver means may comprise a finite impulse response filter or an infinite impulse response filter.
The receiver may be incorporated in a mobile station. Preferably, the estimating means estimates the coherence time of the channel of the propagation path associated with the respective receiver means based on a parameter indicative of the movement of the mobile station. Movement of the mobile station will be an important factor in the changing of the coherence time.
As the coherence time estimate is based on a parameter indicative of the movement of the mobile station, a reasonable estimate of the coherence time can be obtained. The parameter indicative of the movement of the mobile station may be defined by a ratio of a first autocorrelation of a channel impulse response, with no delay, for the propagation path associated with a given receiver means and a second autocorrelation of said channel impulse response with a given delay. This has the advantage that it can be simply implemented using only a few components.
Preferably, the first and second auto correlations are average values. Thus, the effects of any anomalous values can be reduced.
Preferably, the output of the filtering means is used to control a phase alteration applied to the received signals. The phase alteration applied to the received signals allows coherent combining of the signals received via different receiver means. The better that the filtering means is able to reduce the effects of noise, the better the phase alteration applied to the received signal.
Preferably, the estimating means is arranged to receive a plurality of channel impulse responses for said received signal, said channel impulse estimates being used by said estimating means to estimate the channel coherence time.
The receiver may also be incorporated in a base transceiver station. The principal of embodiments of the present invention can be used to compensate for the effects of changes in the radio environment.
Preferably, the receiver is arranged to receive signals in a code division multiple access format.
According to a second aspect of the present invention, there is provided a receiver for use in a wireless communication system, said receiver comprising a plurality of receiver means, said plurality of receiver means each being arranged to receive signals from a different propagation path, each of said receiver means comprising adaptive filtering means for filtering said received signals, wherein the operation of said adaptive filtering means is altered in dependence on a characteristic of the signals from the propagation path received by the respective receiver means.
Since the filtering means in each receiver means is individually altered in dependence on the signal received by the individual receiver means, an optimal performance of the receiver can be achieved.
Preferably, the received signals are processed prior to passing through adaptive filtering means. Preferably, the characteristic is the channel impulse response for the propagation path used by the signals received by the respective receiver.
It should be appreciated that there are features of the first aspect of the invention can be provided with the second aspect and vice versa.